Lymph node cells from high and low responder mice which had been immunized with nuclease were cultured in microtiter plates with nuclease or nuclease fragments in a serum-free culture system. The cultures were pulse labelled for DNA or protein synthesis by incorporation of radioactive thymidine or leucine. Monospecific alloantiserum exhibiting specificity exclusive for H-2 were prepared using congenic B10 and B10.A mice. Other alloantiserum (specific for H-2 and other differences) was prepared in DBA/1 and SJL/J mice. The "blocking" of nuclease-induced lymphocyte stimulation is to be tested by incubating the cells with the antisera prior to culture with nuclease or nuclease fragments.